1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include a plurality of base stations or other devices for providing wireless connectivity over associated geographic areas that are conventionally referred to as cells. Mobile units located in or near cells associated with the base stations may establish wireless communication links over an air interface between the mobile unit and the base station. The properties of the air interface between the mobile units and the base stations are typically defined by industry-wide agreed-upon standards and protocols. One exemplary set of standards and/or protocols is referred to as Orthogonal Frequency Division Multiple Access (OFDMA). In OFDMA systems, the air interface is formed in a carrier frequency band that encompasses a plurality of sub-carrier frequency bands. Each sub-carrier is transmitted in a narrow frequency band centered on a sub-carrier frequency that is orthogonal to all of the other sub-carrier frequencies. Orthogonality of the sub-carrier frequencies permits multiple mobile units to establish concurrent wireless communication links with each base station with minimal inter-carrier interference. Using multiple sub-carriers may also help reduce multipath frequency selective fading of transmission between the mobile unit and the base station.
FIG. 1 conceptually illustrates a conventional process flow 100 for extracting individual user signals from a multi-user super-positioned baseband signal. In the illustrated embodiment, a baseband signal including a superposition of multiple user signals is received at the base station. Cyclic prefixes in the received symbols are removed (at 105) from the baseband signal and then a fast Fourier transform (at 110) is performed to convert the baseband signal to the frequency domain. Resource block demapping and demultiplexing is performed (at 115) to segregate the data traffic signals for each user, uplink control channel, and random access (RACH) channel. A RACH detection process is also performed (at 120) on the baseband signal and the information provided by the RACH detection process is used for downlink control.
FIG. 2 conceptually illustrates a conventional process flow 200 for performing RACH detection. In the illustrated embodiment, a long fast Fourier transform is performed (at 205) on the received baseband signal to convert the received signals to the frequency domain. The RACH signals are extracted (at 210) from the frequency domain signal. A discrete Fourier transform is performed (at 215) on a Zadoff-Chu reference the sequence that corresponds to an expected RACH signal. The transformed Zadoff-Chu sequence is combined (at 220) with the extracted RACH signal and an inverse discrete Fourier transform is performed (at 225) on the combined signals. The results of the inverse discrete Fourier transform of the combined signals may then be provided to a peak detection algorithm to extract out the timing of the initial access signal from the non-synchronized RACH channel. A single FFT processor is commonly used for both data processing and the RACH detection. This approach assumes that the multi-user super positioned signals are perfectly time aligned, but this assumption is not always valid in practice.
A variety of factors may cause a frequency mismatch between the sub-carrier frequencies of signals transmitted by the mobile unit and the signals received at the base station. For example, the Doppler shift caused by relative motion of the mobile unit and the base station may introduce a frequency offset between the sub-carrier frequency of the signals received at the base station and the expected value of the sub-carrier frequency. For another example, inaccuracies in the oscillators used to generate the signal transmitted by the mobile unit and/or the oscillators used to generate the reference signal at the base station may introduce a frequency offset. The frequency offset causes misalignment between the subcarrier center frequency and the fast Fourier transform kernels used to process the received signals. Furthermore, the frequency offsets are typically different for each mobile unit. Consequently, the fast Fourier transform processing on the baseband signal, which operates on received signals that are down converted to the baseband using a local reference oscillator, may convolve signals transmitted on different sub-carrier frequencies and generate inter-carrier interference.
FIG. 3 conceptually illustrates subcarrier frequencies associated with two users. In the illustrated embodiment, each frequency band includes five subcarrier frequency bands 300, 305. The subcarrier frequency bands 300 associated with the first user are indicated by solid lines and the subcarrier frequency bands 305 associated with the second user are indicated by dashed lines. The reference frequencies employed by the receiver for performing various Fourier transforms associated with the subcarrier frequency bands are indicated by boldfaced arrows 310. The center frequency 315 of the first carrier frequency band is offset from the reference frequencies 310 by a frequency offset Δf1 and the center frequency 320 of the second carrier frequency band is offset from the reference frequencies 310 by a frequency offset f2. These frequency offsets for each user create a convolving effect in the fast Fourier transform processing, which generates inter-carrier interference between the carrier/subcarrier frequency bands 300, 305. The inter-carrier interference is approximately proportional to the degree of the frequency offset.
Referring back to FIG. 1, the inter-carrier interference generated by convolution of signals in different subcarrier frequency bands occurs primarily within the resource block 115 following the fast Fourier transform processing 110. Consequently, frequency offset estimation is usually performed using the extracted pilot symbol for each user after the resource block demapping and user demultiplexing 115. For example, a multi-tap filter may be used to compensate for the convolving effect of frequency offsets to reduce the inter-carrier interference. The frequency offset compensation for each pilot and data subcarrier is performed before the channel estimation and equalization, which requires a high complexity deconvolution operation (such as multi-tap filtering) to remove the inter-carrier interference after FFT processing 110. Thus, when deconvolution operations are needed to compensate for the effects of frequency offsets, the conventional techniques for removing inter-carrier interference significantly increase the complexity of the OFDMA baseband processing. Larger frequency offsets require increasing the number of taps to cover more subcarrier frequencies and therefore increasing the complexity of the processing required to perform these operations. Furthermore, the multi-tap filters must be applied to signals associated with each user after the fast Fourier transform processing 110.